This invention relates to gas burner controls for furnaces and the like, and in particular to a new and improved main burner valve control. The system utilizes a pilot burner with valve and valve solenoid, and a main burner with valve and valve solenoid. The system also includes a pilot burner flame sensor and an electronic circuit responding to a sensed flame for switching between a pilot burner ignition condition and a main burner on condition.
The main burner in a gas furnace or other gas powered device normally is ignited by a pilot burner and in the past, the pilot burner was maintained on all the time. However in order to save fuel costs, newer systems are being provided where the pilot burner is normally off and is turned on only when it is desired to turn on the main burner. When the system is to be turned on, such as when a thermostat switch closes, the fuel is supplied to the pilot burner by energizing a pilot valve solenoid. A spark ignitor circuit is also energized producing a spark at the pilot burner for igniting the pilot burner. When a flame is established at the pilot burner, a flame sensor provides a signal to a control circuit which actuates a relay to switch the spark ignitor circuit off and to energize the main burner valve solenoid, turning on the main burner which is ignited by the pilot burner. A control system of this general type is shown in the co-pending application Ser. No. 590,410, filed June 26, 1975 and now U.S. Pat. No. 3,986,813. Other prior art systems are identified in said co-pending application.
If for any reason the pilot burner does not ignite, the main burner valve solenoid will not be energized. Similarly, if the pilot burner flame goes out while the main burner is on, the outage will be noted by the flame sensor with the control circuit turning the main valve burner off and again energizing the pilot spark ignition circuit. This operation protects against the possibility of the main burner valve being open while the pilot flame is out. However there is always a possibility that a component in the electronic circuit will malfunction with the result that the main burner solenoid may be energized even though there is no pilot flame. Accordingly, it is an object of the present invention to provide a new and improved control system which will have a fail safe operation for every component in the main control circuit, that is, if any component fails such as by shorting or open circuiting, the main burner valve cannot be opened. A further object is to provide such a system wherein the control circuit is of the analog-digital type with the flame sensor output being an analog measurement while the remainder of the control operation is digital in nature.